The human T-cell leukemia virus, HTLV-I, has been found to be associated with patients with adult T-cell leukemia. Studies are underway to understand the mechanism of malignant transformation of cells infected with this virus and the immunologic response of individuals who are infected with this virus and who demonstrate malignancies, or those who are carriers of the virus but have not developed malignancies. Chronic lymphocytic leukemia (CLL) cells were obtained from patients who were HTLV seropositive; however, their malignant B-cells did not contain the HTLV-I retrovirus. Using hybridoma technology, CLL cells were fused with a B- lymphoblastoid cell line and the immunoglobin captured. In one instance, the captured immunoglobin reacted with the HTLV-I p24 gag proteins and, in the other instance, the large envelope protein from HTLV-I. Immunoglobin gene rearrangement present in the B CLL cells was demonstrated in the hybridoma cell line. The results indicate that the CLL cells were antigen-committed cells prior to malignant transformation. Sera from an individual who developed a B-cell lymphoma and whose spouse had acute T-cell leukemia (ATL) (HTLV-I positive) contained an antibody with reactivity to HTLV-I- positive cells. The B-cell lymphoma also produced an antibody reactive with HTLV-I when established in short-term culture.